1996A&A...314L...5C

We present the first results of a programme of deep imaging of ultraluminous IR galaxies (ULIRGs) that, have previously been identified as non-interacting and non-merging objects in previous work at optical wavelengths. We find that all six of the observed objects (five at optical wavelengths, one in the IR) show clear signs of disturbance in our deep images. The previous observations may have missed these features through poor seeing, failure to reach faint enough magnitudes, or, in one case, as a result of obscuration in the optical which was penetrated by our longer wavelength IR observations. We conclude that it is likely that the vast majority of ULIRGs have been triggered by merging or interaction events, though the effects of these events may be difficult to detect in some objects.